This invention relates to a projection optical system, an exposure apparatus, and an exposure method. More specifically, this invention relates to a projection optical system that is preferably used in an exposure apparatus that manufactures devices such as a semiconductor element, using EUV light, in a photolithographic process.
Conventionally, in an exposure apparatus used in manufacturing a semiconductor element or the like, a circuit pattern formed on a mask (reticle) is projected and transferred onto a photosensitive substrate (for example, a wafer) via a projection optical system. A resist is coated on the photosensitive substrate, the resist is photosensitized by projection exposure via the projection optical system, and a resist pattern corresponding to a mask pattern is obtained.
Here, resolution W of an exposure apparatus depends on a wavelength λ of exposure light and a numerical aperture NA of the projection optical system and is shown by the following equation (a).W=k·λ/NA(k:constant)  (a)
Therefore, in order to improve the resolution of the exposure apparatus, the wavelength λ of the exposure light is shortened, and the numerical aperture NA of the projection optical system needs to be increased. In general, increasing the numerical aperture NA of the projection optical system to a predetermined value or more is difficult from a perspective of optical design, so exposure light needs to have a shorter wavelength.
Therefore, as an exposure method (exposure apparatus) for the next generation of semiconductor patterning, an EUVL (Extreme Ultra Violet Lithography) method has been focused upon. In the EUVL exposure apparatus, compared to a conventional exposure method using a KrF excimer laser beam in which the wavelength is 248 nm and an ArF excimer laser beam in which the wavelength is 193 nm, EUV (Extreme Ultra Violet) light having a wavelength of approximately 5-20 nm is used.
When EUV light is used as exposure light, an optically transmissive material that can be used does not exist. Because of this, in the EUVL exposure apparatus, a reflective type mask is naturally used, and a reflective type projection optical system is used.